Almost all of the commercially available carbon fibers are produced from a polyacrylonitrile fiber. Those carbon fibers are very expensive because the polyacrylonitrile fiber as the raw material is expensive and the yield of the carbon fiber is low. In contrast to the conventional process, a specific attention is recently drawn to a process for producing the carbon fiber from a carbonaceous pitch as a process for producing an inexpensive carbon fiber because the raw material is inexpensive and the yield of carbon fiber is high.
One of the important features required in the mesophase pitch used as the raw material for carbon fiber is that the mesophase pitch has an excellent spinnability. To achieve this requirement, it is desired for the mesophase pitch to be the content of quinoline-insoluble high molecular weight components small. Thus, a process for producing a mesophase pitch containing quinoline-soluble low molecular weight mesophase is demanded.
Unexamined Published Japanese Patent Application No. 160427/1979 discloses a process for producing a quinoline-soluble mesophase by extracting an isotropic pitch with a solvent and heating the insolubles at 230.degree. to 400.degree. C.
Unexamined published Japanese Patent Application No. 55625/1979 discloses a process for producing a mesophase pitch containing a pyridine-soluble mesophase by heat treating with stirring an isotropic pitch at 380.degree. to 430.degree. C. in an inert gas stream.
Unexamined published Japanese Patent Application No. 57881/1981 discloses a process for producing a mesophase pitch containing a pyridine-soluble mesophase by subjecting a pitch to physical operations such as a solvent extraction.
Unexamined published Japanese Patent Application No. 101915/1981 discloses a process for producing a mesophase pitch containing a pyridine-soluble mesophase by heat treating a pitch precursor such as ethylene tar at 400.degree. to 550.degree. C. under pressure and then heat treating the same in an inert gas atmosphere under the atmospheric pressure.
Unexamined published Japanese Patent Application No. 42924/1982 discloses a process for producing a mesophase pitch by heat treating with stirring a pitch precursor under the atmospheric pressure and then heat treating the same with stirring in an inert gas stream.
Unexamined published Japanese Patent Application No. 119984/1982 discloses a process for producing a mesophase pitch by heat treating a catalytic cracking by-produced tar at 380.degree. C. or more to produce a mesophase in an amount of 20 to 80%, allowing to stand the resulting product at 400.degree. C. or less to precipitate the mesophase at the lower layer and separating the same.
Unexamined published Japanese Patent Application Nos. 168987/1982 and 168988/1982 disclose a process for producing a mesophase pitch by adding a partially hydrogenated product of two-ring or three-ring aromatic hydrocarbons to a heavy oil formed by steam cracking and catalytic cracking of petroleums, heat treating at 370.degree. to 480.degree. C. and then heat treating at 340.degree. to 450.degree. C. under normal pressure or reduced pressure in an inert gas stream.
Unexamined published Japanese Patent Application Nos. 168989/1982 and 168990/1982 disclose a process for producing a mesophase pitch by treating the same raw material as described above at 400.degree. to 500.degree. C. under pressurized hydrogen and then heat treating in the same manner as described above.
Unexamined published Japanese Pat. Nos. 170990/1982, 179285/1982, 179286/1982, 179287/1982, and 179288/1982 disclose a process for producing a mesophase pitch by adding to the same raw material as described above hydrogenated products of the fractions having a boiling point of 160.degree. to 400.degree. C. which are formed in various steps in the course of the production thereof, treating at 370.degree. to 480.degree. C., and performing the same heat treatment as above.
Other than these processes for producing a mesophase pitch, the following methods for the improvement of spinnability of mesophase pitch are proposed.
According to Unexamined Published Japanese Patent Application No. 100186/1982, a latent anisotropic pitch is obtained by hydrogenating a mesophase pitch with an alkali metal.
According to Unexamined Published Japanese Patent Application No. 18421/1983, a premesophase is obtained by reacting a pitch with tetrahydroquinoline at 300.degree. to 500.degree. C. and then heat treating at 450.degree. C. or more under reduced pressure for a short period of time.
The mesophase pitches thus obtained are all optically isotropic but become anisotropic in the spinning step and the subsequent steps to give a high-performance carbon fiber.
Thus, the conventional processes involve the disadvantages that the steps are complicated, the treatment time is long and an expensive reagent is used.